1. Field of the Invention
The present invention generally relates to exploration and measurements made downhole in a borehole.
2. Description of the Related Art
Boreholes are drilled deep into the earth for many applications such as carbon dioxide sequestration, geothermal production, and hydrocarbon exploration and production. In all of the applications, the boreholes are drilled such that they pass through or allow access to a material (e.g., a gas or fluid) contained in a formation located below the earth's surface. Different types of tools and instruments may be disposed in the boreholes to perform various tasks and measurements.
In more detail, wellbores or boreholes for producing hydrocarbons (such as oil and gas) are drilled using a drill string that includes a tubing made up of, for example, jointed tubulars or continuous coiled tubing that has a drilling assembly, also referred to as the bottom hole assembly (BHA), attached to its bottom end. The BHA typically includes a number of sensors, formation evaluation tools, and directional drilling tools. A drill bit attached to the BHA is rotated with a drilling motor in the BHA and/or by rotating the drill string to drill the wellbore. While drilling, the sensors can determine several attributes about the motion and orientation of the BHA that can used, for example, to determine how the drill string will progress. Further, such information can be used to detect or prevent operation of the drill string in conditions that are less than favorable.
In the process of extracting hydrocarbons, e.g., petroleum, from beneath the surface of the earth, wells are drilled and steel pipe (casing) is placed into the drilled hole (wellbore or borehole). Cement is then pumped into the annular space between the casing and the rock wall of the borehole (formation). The cement serves two major purposes. First, it transfers stress from the casing to the formation, increasing the effective strength and working pressure of the casing. Second, it serves to isolate vertically adjacent zones within the formation, preventing migration of liquids and gases up the wellbore between the formation and the casing, from one zone to the next. Accordingly, it is important to assure the quality of the cement both during completion of the well and during its producing life. Acoustic cement evaluation devices (e.g., cement bond logs (“CBL”)) conveyed by a cable (wireline) to move the tools up and down in the borehole have been the primary device to provide this assurance. The principle of evaluation is based on the loss of energy over time or distance of an acoustic wave excited in the casing. One area of conventional focus has been on determining whether cement is present outside the casing, e.g., to determine whether only fluid is present between the casing and the formation (free pipe), whether the space between the casing and the formation is filled with cement bonded to the casing (fully cemented pipe), or whether a small gap, filled with fluid, exists between the cement and casing (micro-annulus). However, conventional techniques are unable to adequately evaluate properties of the cement.